1. Field of the Invention
The subject invention pertains to environmentally durable lightning strike protection materials for use in composite structures, particularly aircraft. More particularly, the subject invention pertains to lightning strike materials which not only provide lightning strike protection, but which maintain this protection even after severe repetitive thermal cycling and exposure to corrosive atmospheres.
2. Description of Related Art
The desirability of protection of structures from lightning dates at least to medieval times, and lightning rods and the use of heavy copper cables to ground radio and television antennas is well known. However, many modern structures are incapable of using grounding as a method of channeling away the intense electrical energy from a lightning strike. Aircraft, for example, must rely on the ability to rapidly dissipate such energy by rapidly distributing it throughout the structure.
In the past, aircraft, automobiles, and other metal structures provided a low resistance pathway throughout the bulk of the structure as a means of energy dissipation. However, the use of composite materials, such as thermosetting and thermoplastic polymer impregnated fiber reinforcement has created problems in this respect due to the much lower electrical conductivity of such materials. Carbon/graphite fiber reinforced materials, while offering higher conductivity than their fiberglass or high temperature thermoplastic reinforced analogues, are still deficient in this respect. Moreover, at surfaces where composite parts are joined together, the electrical resistance is often extremely high.
To compensate for these deficiencies of modern fiber reinforced structural materials, designers have sought to incorporate conductive pathways of metal throughout or along the surfaces of the structure in order to provide a means of rapidly dissipating the energy received in a lightning strike. The performance of such strategies may be assessed by exposing the protected structure to high energy electrical discharges. Several such tests are required for many applications. These tests are more fully described infra.
Many methods now presently exist to provide adequate lightning strike protection. However, commercially acceptable methods must be of relatively low cost, lightweight, and durable. With respect to the latter quality, since aircraft often operate in moist salty air environments or are subject to salt spray, corrosion of the lightning strike conductors can cause a loss of protective ability, and can also lower the strength of the composite structure itself.
The use of metal screens has been proposed for use as lightning strike protective materials. Such screens may be prepared by several methods. Expanded metal screens, for example, may be prepared by slitting metal sheet in a geometric pattern followed by stretching the sheets in a direction transverse to the slit orientation. Suitable screens may also be prepared by chemically etching holes in metal foils, by perforating metal foils using high energy electromagnetic beams, for example laser beams, by use of plasma beams, etc. Screens may also be formed by weaving metal filaments into a gauze-like structure.
By whatever method the screen is formed, it must be of sufficient conductivity to dissipate electrical energy, but must be of sufficient lightness to avoid excessive weight, particularly in military and commercial aircraft. Moreover, to avoid corrosion, the screen must be completely encased in a non-porous covering which is further not susceptible to microcracking which could allow for penetration of compositions which may promote corrosion.
Previous methods of providing for lightning strike protection included the bonding of a metal screen, such as those described previously, to the substrate for which protection is desired, by means of a film adhesive, either as a neat film or supported by a thin ply of fabric (scrim) such as lightweight polyester. The composite structure, screen, and adhesive are then co-cured. Unfortunately, such assemblies, upon cure, failed to provide complete coverage of the screen or were not durable, developing multitudinous microcracks after only a limited amount of thermal cycling. Such attempts also failed to provide the requisite amount of corrosion resistance.